This invention relates to resonant inverters and more particularly, to a resonant inverter with improved control allowing it to provide a substantially constant output voltage to a load over a wide range of operating conditions.
Operating dc-ac inverters with a resonant circuit is attractive since sine wave currents and voltages can be achieved, and since inverter switching devices can operate with low switching losses. Another advantage of using resonant inverters is the potential for obtaining reduced electromagnetic interference due to the low harmonic content of the sinusoidal current and voltages. One disadvantage of using resonant inverters, however, is that when input power or output load conditions are varying, output voltage or current control may not be achieved through the use of usual control techniques. This is because of the resonant nature of the circuit.
One known resonant inverter output load voltage or current control method is to vary the frequency of the rectangular wave signal supplied to the resonant circuit by the inverter via closed loop control. Commonly assigned U.S. Pat. No. 4,541,041, of Park et at., which is incorporated herein by reference, discloses in part such a frequency control technique. Briefly explained, the resonant nature of the circuit allows for control of output voltage or current through variation of the frequency at which the inverter's controllable switch means operate. Such a frequency control method has been found satisfactory under normal output load conditions for particular types of resonant inverters (i.e. heavy or medium load conditions for a series resonant inverter and light load conditions for a parallel resonant inverter). The drawback to frequency control, however, is that it may be inadequate to maintain a desired output voltage or current under extended output load conditions (i.e., light or no load conditions for a series resonant inverter and heavy load conditions for a parallel resonant inverter).
By way of example, frequency control of a series resonant inverter will normally be adequate to maintain a desired output voltage during heavy or medium load conditions (i.e., low resistance) because for heavy or medium load conditions, a series resonant circuit has high Q and thus a good dynamic range of voltage or current change as frequency is varied. However, under extended or light output load conditions (i.e., high resistance), the series resonant circuit exhibits a low Q and thus a small dynamic range of voltage or current change as a function of frequency. As a result, in the case of the series resonant inverter, it may be impossible to maintain a desired output voltage or current under light load and no load conditions solely with frequency control.
Therefore, there presently exists a need for a resonant inverter control which, under extended load conditions, provides an improved dynamic range of output voltage or current control than that which can be provided solely by closed loop frequency control.